1. Field of Invention
The invention relates to a method of applying a pattern to a surface of a substrate. In this method, mutually parallel pattern rows are applied by at least two output channels.
2. Description of Background Art
Methods for applying a pattern are used, inter alia, during the production of printing stencils. In this case, for example during the production of a cylindrical flexographic printing plate, an opaque covering liquid is sprayed onto the photosensitive elastomeric layer of a blank with the aid of spray nozzles, which form the output channels. The spray nozzles are driven by a central computing unit such that the covering liquid is sprayed on in accordance with the pattern to be produced.
To apply a surface-covering pattern in the conventional method, a spray head which has a number of spray nozzles is used where a large number of pattern rows which are located immediately alongside one another are produced by each spray nozzle, starting from a first position. The spray head is respectively displaced by one pattern row spacing. In each case, during the application of a pattern row or thereafter, until the last pattern row sprayed on by a specific spray nozzle is located immediately alongside the first pattern row of the next spray nozzle in the direction of displacement, the spray head is displaced. In this way, a pattern section is produced whose width in the direction of displacement is equal to the product of the distance of the spray nozzles from one another and the number of spray nozzles. This pattern section is composed of a large number of pattern strips, corresponding to the number of spray nozzles, each strip being formed from a large number of pattern rows which have all been produced by one and the same spray nozzle.
After a pattern section has been applied, the spray head is displaced by the distance between the first and last spray nozzles in order to produce the next pattern section in the manner described.
However, since the individual spray nozzles in the spray heads are not identical and since, therefore, the size and velocity of the drops sprayed and/or the spraying direction may fluctuate, for example within specific tolerance limits, each pattern row produced by a spray nozzle exhibits properties which are different from those of a pattern row which was produced by another spray nozzle. However, since all of the pattern rows of one pattern strip are produced by one and the same spray nozzle, they should also have the corresponding characteristic, which is more clearly visible as a result of this repetition.
In particular, fluctuations in the size of the drops sprayed by the individual spray nozzles have a noticeably disturbing effect as fluctuations in the grey value, for example in the case of screened continuous-tone patterns.
The result is, in particular, a striation of the pattern in the direction of the pattern rows, that is to say, in the case of a pattern applied in the circumferential direction to a cylinder, in the circumferential direction of the cylinder.
The connecting point between the individual pattern sections is particularly problematic, since in the case of spray heads having a number of spray nozzles, the differences between the individual nozzles are such that, although they are not very great from one spray nozzle to the next one, they become greater from one end of the spray head towards the other. Thus, the difference between the first and last spray nozzle is typically the greatest. Since, in the connecting region of the pattern sections, the pattern strip produced by the first spray nozzle immediately abuts a pattern strip produced by the last spray nozzle, the differences are particularly clearly noticeable at this location.
This problem becomes particularly clear in the case where a spray nozzle fails completely and does not produce a pattern strip. In the case of a typical spray-nozzle spacing of about 1 mm, the pattern then has pattern-free strips at regular intervals and, at 1 mm wide, these strips stand out clearly.
The problems described occur not only in the case of spray nozzles for applying covering liquids or the like, but also in the case of other output channels which operate with laser light, for example, in order either to expose a photosensitive layer in accordance with a pattern or to remove a varnish layer or the like in accordance with a pattern by means of evaporation or the like.